1. Field of the Invention
This invention relates to an optical scanning device using a hologram.
2. Description of Related Art
FIG. 10 shows one type of related art optical scanning device for modulating a laser beam in accordance with an image signal and irradiating the modulated laser beam onto a photosensitive medium to form an electrostatic latent image on the medium. The related art image forming device shown in FIG. 10 comprises a semiconductor laser 101, a collimator lens 102 for collimating a laser beam emitted from the semiconductor laser 101, a polygon mirror 103 for reflecting the collimated laser beam from the collimator lens 102 while rotating at high speed, an f.theta. lens 104 for converging the beam reflected from the polygon mirror 103 and a return mirror 106 for reflecting the beam from the f.theta. lens 104 to a photosensitive drum 105 to form an image on a photosensitive drum 105.
FIG. 11 shows the P-I (optical power vs. forward current) characteristic of the semiconductor laser 101 used in such a related art optical scanning device. As shown in FIG. 11, the P-I characteristic shows a faster increase in optical power with forward current as the forward current increases above a threshold current value of I.sub.B1. The forward current at which the P-I characteristic changes is determined by the ambient temperature. In FIG. 11, the optical power output level P.sub.S1 is the minimum power level at which an electrostatic latent image can be formed for a particular photosensitive medium. That is, upon irradiation of a laser beam having a power level above the latent image forming level P.sub.S1 onto the photosensitive medium, the potential of a beam-incident area is reduced to such a level that development with toner can be performed (that is, toner can be attracted and attached to the area). Conversely, if the incident laser beam power is less than the latent image forming level P.sub.S1, the potential reduction on the photosensitive medium is insufficient for toner to be attached to the beam-incident area.
Representing an optical power level which is high enough to reduce the potential on the photosensitive drum 105 to a toner-attachable level (hereinafter referred to as "on-level") by P.sub.1, the current I.sub.1 flowing at the optical power level P.sub.1 is larger than the current I.sub.S1 corresponding to the optical power P.sub.S1. Additionally, a current whose value is near to the threshold current value I.sub.B1 flows as a bias current into the semiconductor laser 101 to compensate for fluctuation of the optical power due to the temperature and to improve stability of an oscillation waveform. A weak optical power occurs with the bias current. This weak optical power P.sub.B1 is not enough to reduce the potential on the photosensitive medium to the toner-attachable level. An optical power level which does not reduce the potential enough for toner to be attached is referred to as at the "off-level".
The optical power of the semiconductor laser 101 is switched between the on-level and the off-level in accordance with an image signal to form an electrostatic latent image on the surface of the photosensitive drum 105. The electrostatic latent image formed on the surface of the photosensitive drum 105 is developed with toner and then transferred onto a sheet.
When the photosensitive drum 105 has high sensitivity and an optical level at which the latent image can be formed is set to P.sub.S2 which is lower than P.sub.S1, the on-level is set to P.sub.2 which is larger than P.sub.S2. On the other hand, the off-level is not varied, that is, it is set to P.sub.B1 because the bias current of the semiconductor laser 101 is set to a value near to the threshold value. However, even in an off-level area subject to the off-level power, toner attachment may occur and occurs more the lower the ratio of P.sub.S2 to P.sub.B1. Accordingly, since the ratio of the latent-image formable optical level P.sub.S2 to the off-level P.sub.B1 (P.sub.S2 /P.sub.B1) is small when the photosensitive drum 105 has high sensitivity, the possibility of the toner attachment increases even when the optical power is at the off-level, so that toner may be attached to an area which is originally white on a reproduced image. Therefore the problem of fog is greater when the photosensitive drum has high sensitivity. Additionally, since the ratio of the on-level to the off-level (P.sub.2 /P.sub.B1) is also small, there is also a problem that the contrast of a reproduced image is low.
Furthermore, even when the photosensitive drum 105 has high sensitivity, the on-level optical power setting remains at P.sub.1 to prevent reduction in resolution due to excessive exposure. A light absorption filter is inserted into a transmission path of the laser beam to reduce the "fog" phenomenon and improve the contrast or the reflectivity of the return mirror 106 is reduced to reduce the optical power to P.sub.2. However, in that case, optical elements such as a light absorption filter, etc. must be added, and a surface coating treatment must be provided to reduce the reflectivity of the return mirror 106. That increases the cost of the device.